1. Field of the Invention
The invention relates to a back-lit operating unit for construction machinery, in particular road milling machines, recyclers, or stabilizers.
2. Description of the Prior Art
Construction machines of different designs for civil engineering and excavation work are well-known. As road-building machines, for example, milling machines, recyclers, stabilizers, or surface miners are known. The known construction machines are provided with one or more operating stands with an operating unit, in order to be able to operate the construction machine.
The operating units of known construction machines, which are subjected to relatively great mechanical stresses, are in general provided with a rigid front panel, in which the components required for the operating of the machine, such as buttons, switches or joysticks etc. are located. These components for operating the machine are clamped or screwed to the front panel.
The known back-lit operating units are provided with a front panel which is transparent in certain areas, which is lit from behind. As a result, the transparent areas appear as light zones, while the remainder of the panel is dark. Front panels are known for back-lit operating units which are adhesively bonded to a printed and essentially transparent film, on which the desired graphic representations are printed as negatives. To increase the contrast, the film can be provided on its rear in the areas which are not back-lit with an aluminium blocking layer. The intention of the blocking layer is to prevent the areas which are not back-lit from showing through when the front panel is lit from behind.
The principle is known of manufacturing the front panel from aluminium sheet and of providing cut-outs in the back-lit areas. This proves to be disadvantageous, since the film impinges in the areas of the cut-outs, which becomes visually evident. In addition to this, the film functions at this point like a membrane, which is not adequate for the mechanical loads imposed. If the membrane is broken, for example, fluid may penetrate and the front panel must be repaired, incurring effort and expenditure.
The principle is already known of manufacturing the front panel from transparent material, such as polycarbonate. Due to the high mechanical loads which may take effect on a front panel, the material thickness in this case must be selected as many times greater than with the use of aluminium sheet. The conventional commercial components for the operation of the machine, such as switches, buttons, or joysticks, predetermine a specified material thickness for the front panel, however, in order for the components to be fitted. This derives from the dimensioning of the securing elements, such as the thread length with screw connections or the distance interval between the locking elements with snap connections. Accordingly, when polycarbonate panels are used with too great a material thickness, simple passage apertures are insufficient, and offset apertures must be provided, which are expensive and elaborate to produce.